During B Cell differentiation, antibody genes are diversified by a number of distinct mechanisms. At early stages of B-cell development, V(D)J recombination leads to an enormous diversity of lg V-region sequences among individual maturing B cells. During later, antigen-dependent, stages of B cell differentiation, another mechanisms, somatic hypermutation, gives rise to additional antibody diversity. Somatic hypermutation appears to be central in allowing the body to produce high-affinity antibody responses to protect against pathogens. On the other hand, somatic hypermutation has also been implicated in aberrantly activating oncogenes involved in tumorigenesis, and, in some B-cell leukemias, somatic hypermutation is apparently ongoing, leasing to tumor cell variations that can defeat therapies based on anti- idiotypic reagents. Very little is known about the molecular mechanisms that are involved in the somatic hypermutational process. We have recently found that sequence transfers which resemble gene conversion events can occur within a murine antibody H-chain transgene and that these sequence transfers correlate with the somatic hypermutation process. We now believe that gene conversion mechanisms could be a part of the somatic hypermutational mechanism. During the proposed project period, we wish to investigate the targeting and mechanism of transgene sequence transfers and determine the types of B cells that undergo sequence transfer events. We will also further investigate the relationships between the sequence transfer and somatic hypermutation processes. The goals of the current proposal are (1) to determine whether transgene sequence transfers occur by gene conversion or DNA recombination, (2) to investigate the effect of spacing on the frequency of transgene conversion between VDJ segments, (3) to localize and characterize those B cells which have undergone transgene conversion and investigate the correlation of transgene conversion and somatic hypermutation, and (4) to investigate sequence transfer among antibody V(D)J segments within the endogenous murine lgH locus. We expect that these studies will indicate whether gene conversion might have a role in the diversification of normal antibody genes and whether gene conversion is involved in the currently uncharacterized mechanism of somatic hypermutation. We hope that a more thorough understanding of the hypermutational process could lead to more effective approaches to treatment of some B cell cancers and to a fuller understanding of some steps of tumorigenesis.